Nature and evolution of the fusion boundary in ferritic-austenitic dissimilar weld metals, Part 1 - Nucleation and growth

A fundamental investigation of fusion boundary microstructure evolution in dissimilar-metal welds (DMWs) between ferritic base metals and a face-cente red-cubic (FCC) filler metal was conducted. The objective of the work prese nted here was to characterize the nature and character of the elevated-temp erature fusion boundary to determine the nucleation and growth characterist ics of DMWs. Type 409 ferritic stainless steel and 1080 pearlitic steel wer e utilized as base metal substrates, and Monel 170Ni-30Cu) was used as the filler metal. The Type 409 base metal provided a fully ferritic or body-cen tered-cubic (BCC) substrate at elevated temperatures and exhibited no on-co oling phase transformations to mask or disguise the original character of t he fusion boundary. The 1080 pearlitic steel was selected because it is aus tenitic at the solidus temperature, providing an austenite substrate at the fusion boundary. The weld microstructure generated with each of the base m etals in combination with Monel was fully austenitic. In the Type 409/Monel system, there was no evidence of epitaxial nucleation and growth as normally observed in homogenous weld metal combinations. The fusion boundary in this system exhibited random grain boundary misorientat ions between the heat-affected zone (HAZ) and weld metal grains. In the 108 0/Monel system, evidence of normal epitaxial growth was observed at the fus ion boundary, where solidification and HAZ grain boundaries converged. The fusion boundary morphologies are a result of the crystal structure present along the fusion boundary during the initial stages of solidification. Base d on the results of this investigation, a model for heterogeneous nucleatio n along the fusion boundary is proposed when the base and weld metals exhib it ferritic (BCC) and FCC crystal structures, respectively.